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  <div class="toc-header"><strong>Contents</strong></div>
<ul>
  <li><a href="./Attribute::Constant.html#NAME">NAME</a></li>
  <li><a href="./Attribute::Constant.html#VERSION">VERSION</a></li>
  <li><a href="./Attribute::Constant.html#SYNOPSIS">SYNOPSIS</a></li>
  <li><a href="./Attribute::Constant.html#DESCRIPTION">DESCRIPTION</a></li>
  <li><a href="./Attribute::Constant.html#ATTRIBUTES">ATTRIBUTES</a></li>
  <li><a href="./Attribute::Constant.html#CAVEAT">CAVEAT</a>
    <ul>
      <li><a href="./Attribute::Constant.html#Multi-line-attributes">Multi-line attributes</a></li>
      <li><a href="./Attribute::Constant.html#Constants-from-Variables">Constants from Variables</a></li>
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<div class="pod anchors">
<h1 id="NAME">NAME</h1>

<p>Attribute::Constant - Make read-only variables via attribute</p>

<h1 id="VERSION">VERSION</h1>

<p>$Id: Constant.pm,v 1.1 2013/04/03 14:37:57 dankogai Exp $</p>

<h1 id="SYNOPSIS">SYNOPSIS</h1>

<pre><code> use Attribute::Constant;
 my $sv : Constant( $initial_value );
 my @av : Constant( @values );
 my %hv : Constant( key =&gt; value, key =&gt; value, ...);</code></pre>

<h1 id="DESCRIPTION">DESCRIPTION</h1>

<p>This module uses <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a> to make the variable read-only. Check the document and source of <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a> for its mechanism.</p>

<h1 id="ATTRIBUTES">ATTRIBUTES</h1>

<p>This module adds only one attribute, <code>Constant</code>. You give its initial value as shown. Unlike <a href="Readonly.html">Readonly</a>, parantheses cannot be ommited but it is semantically more elegant and thanks to <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a>, it imposes almost no performance penalty.</p>

<h1 id="CAVEAT">CAVEAT</h1>

<h2 id="Multi-line-attributes"><a id="Multi"></a>Multi-line attributes</h2>

<p>Multi-line attributes are not allowed in Perl 5.8.x.</p>

<pre><code>  my $o : Constant(Foo-&gt;new(one=&gt;1,two=&gt;2,three=&gt;3));    # ok
  my $p : Constant(Bar-&gt;new(
                            one   =&gt;1,
                            two   =&gt;2,
                            three =&gt;3
                           )
                 ); # needs Perl 5.10</code></pre>

<p>In which case you can use <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a> instead:</p>

<pre><code>  dlock(my $p = Bar-&gt;new(
        one   =&gt; 1,
        two   =&gt; 2,
        three =&gt; 3
    )
  );</code></pre>

<p>After all, this module is a wrapper to <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a>;</p>

<h2 id="Constants-from-Variables"><a id="Constants"></a>Constants from Variables</h2>

<p>You may be surprised the following code <b>DOES NOT</b> work as you expected:</p>

<pre><code>  #!/usr/bin/perl
  use strict;
  use warnings;
  use Attribute::Constant;
  use Data::Dumper;
  {
    package MyClass;
    sub new {
        my ( $class, %params ) = @_;
        return bless \%params, $class;
    }
  }
  my $o = MyClass-&gt;new( a =&gt; 1, b =&gt; 2 );
  my $x : Constant($o);
  print Dumper( $o, $x );</code></pre>

<p>Which outputs:</p>

<pre><code>  $VAR1 = bless( {
                 &#39;a&#39; =&gt; 1,
                 &#39;b&#39; =&gt; 2
               }, &#39;MyClass&#39; );
  $VAR2 = undef;</code></pre>

<p>Why? Because <code> $x : Constant($o) </code> happens <b>before</b> <code>$o = Myclass-&gt;new()</code>.</p>

<p>On the other hand, the following works.</p>

<pre><code>  my $y : Constant(MyClass-&gt;new(a =&gt; 1,b =&gt; 2));
  print Dumper( $o, $y );</code></pre>

<p>Rule of the thumb is do not feed variables to constant because varialbes change after the attribute invocation.</p>

<p>Or simply use <code>Data::Lock::dlock</code>.</p>

<pre><code>  use Data::Lock qw/dlock/;
  dlock my $z = $o;
  print Dumper( $o, $y );</code></pre>

<h1 id="SEE-ALSO"><a id="SEE"></a>SEE ALSO</h1>

<p><a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a>, <a href="https://metacpan.org/pod/constant">constant</a></p>

<h1 id="AUTHOR">AUTHOR</h1>

<p>Dan Kogai, <code>&lt;dankogai+cpan at gmail.com&gt;</code></p>

<h1 id="BUGS-&amp;-SUPPORT"><a id="BUGS"></a><a id="BUGS---SUPPORT"></a>BUGS &amp; SUPPORT</h1>

<p>See <a href="https://metacpan.org/pod/Data::Lock">Data::Lock</a>.</p>

<h1 id="ACKNOWLEDGEMENTS">ACKNOWLEDGEMENTS</h1>

<p><a href="Readonly.html">Readonly</a></p>

<h1 id="COPYRIGHT-&amp;-LICENSE"><a id="COPYRIGHT"></a><a id="COPYRIGHT---LICENSE"></a>COPYRIGHT &amp; LICENSE</h1>

<p>Copyright 2008-2013 Dan Kogai, all rights reserved.</p>

<p>This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.</p></div>

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        <p>To install Data::Lock, copy and paste the appropriate command in to your terminal.</p>
        <p><a href="https://metacpan.org/dist/App-cpanminus/view/bin/cpanm">cpanm</a></p>
        <pre><code>cpanm Data::Lock</code></pre>
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install Data::Lock</code></pre>
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